4 - LANs Flashcards
Conventional Channel Allocation (2)
- Polling: Offers channel to individual user for fixed amount of time
- Frequency and Time Division Multiplexing Access
Pure ALOHA Method (3)
- Users transmit whenever they wish
- Users detect there own collisions
- After collision, user backoff for a random time period then retransmits
Expectations of ALOHA (3)
- Inf number of users
- Generation of packets is a poisson distribution, with mean S
- Best utilisation is 18.38%
Slotted ALOHA Method (3)
- Satellite generates timing pulse
- Users only transmit when they receive pulse
- Best utilisation 36.79%
LAN Definition
Local Area Network
“ A LAN is a routerless network, using the same protocol stack for each device and using only a uniform, local, networking media”
CSMA
Carrier Sense Multiple Access
Factors Affecting CSMA LANs (9)
- Constant frame size
- No transmission erros
- No capture effect
- Randomly delay after a collision is uniform distr
- Frame gen has poisson distr
- Station cannot trans and rec at the same time
- Each station can see transmission of other stations
- Sensing of channel can be performed at same time as trans
- propagation delay small compared to frames’ trans time
1-persistance CSMA protocols are no good with..
satellite broadcasting due to latency
non & p-persistant CSMA protocols
If channel is too busy, station backs off for various intervals
Channel approaches max, but each station can be waiting a long time.
Ethernet Physical Properties (4)
- Packet at least 64 bytes long
- Segments can’t be bigger than 500m
- Faster twisted pair speeds 10GBase -T
- Fiber Optic speeds
Eth Contention Algorithm, on detection of collision (4)
Known as Binary Exp Back-Off
- backs off for a random period, multiple of 8023 slot time
- After collision, each station backs off for 0 or 11 slot times before trying again
- In general, a station backs off from 0 to 2^(i-1) slot times after i^th collision
- After 16 collisions, station considers the ‘ether’ severed
Ethernet Addr Layout
Preamble -> SD -> Dest -> Src -> Length -> Data -> CRC
Ethernet Addr Scheme (4)
- If all 48 dest bits set to 1, packet is broadcasted on LAN
- MSB: 0 - individual, 1- multicast addr
- Bit 46 used to indicate for current or global LAN
- Uses 2^46 bits, therefore every device has unique ethernet card
Packet Transport Mechanisms (5) used by 802.3
- carrier detection: eth uses carrier sense mechanism of phase encode, guarantee at least one phase transition
- packet error detection
- interference detection
- truncated packet filtering
- collision consensus enforcement: detection of collision by station causes station to ‘jam’ ether so that other stations know
Interference Detection Advantages (3)
- station can detect collisions and re-schedule
- interference is detected within the propagation time
- frequency of collisions is immediately used to dynamically change the back-off times
